Treatment of polymers containing cyano groups with amines and hydrogen sulfide



Patented Aug. 7, 1951 TREATMENT OF POLYMERS CONTAINING CYANO GROUPS WITHAMINES AND BY- DROGEN SULFIDE George Moore Rothrock, Kenmore, N. Y.,assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation of Delaware No Drawing. Application May 2, 1947, Serial No.745,632

Claims.

This invention relates to the chemical modification of structuresprepared from synthetic polymers and particularly to the treatment ofstructures prepared from polymers containing nitrile groups. Bystructures" is meant any form of the polymer, for example, yarns,threads, fabrics, films or the like.

It is well known that the physical and chemical properties of polymersmay be materially and advantageously modified by cross-linking andvulcanizing techniques. One example is the preparation of rubber-likecompounds from polyvinyl alcohol or its esters by heating the polymericmaterials in the presence of sulfur or sulfur compounds. Such processes,however, o not disclose satisfactory means for after-treating formedstructures of nitrile containing polymers by modification of orreactions involving the nitrile groups. Fibers and films prepared fromhigh molecular weight vinyl polymers containin a nitrile group, which inthe monomeris adjacent to the vinyl linkage, are not unusually heatstable. Such materials generally lose strength rapidly when exposed forlong periods of time to heat, for example. at temperatures varying from150 C. to 250 C. Furthermore, if the materials are under tension duringthe exposure, excessive elongation or creep occurs. Fabrics prepared'from vinyl polymers containing such nitrile groups, for example,polyacrylonitrile or similar polymers, have good dielectric propertiesand also a general intertness to moisture, fungus, oilsand commonorganic solvents. Furthermore, they have excellent electricalresistivity and inertness to chemicals. However, their relatively lowheat distortion temperatures preclude their extensive use in suchapplications as electrical laminations, electrical insulation, hightemperature filtrations, high-speed power and conveyor belts and otheruses in which articles are exposed to considerable heat. Since thematerials prepared from nltrile containing vinyl polymers have excellentdielectric properties as well as general stability, a solution to theproblem. of thermal in-. stability is desirable.

An object of this invention is the provision of a method for improvingcertain physical properties of fabrics and the like of nitrilecontaining polymers. A further object is the provision of such polymericstructures having improved heat stability. A still further object is theprovision of a method for raising the melting point of low melting,easily spinnable polymerswhich contain nitrile groups with theproduction of materials having improved dimensional stability at 2elevated temperatures and which retain the excellent dielectricproperties of the original structures such as yarns, fabrics, or thelike. Other objects will appear hereinafter.

The objects of this invention can be obtained through controlledchemical treatments of fabrics, yarns or the like prepared from polymerscontaining a plurality of nitrile groups attached to the polymer chain,as, for example, polymers resulting upon the polymerization,copolymerization, or interpolymerization of vinyl compounds having anitrile group adjacent to the vinyl linkage. The preferred method ofachieving the purposes of this invention is to heat the polymericstructure, for example, the fabric, yarn or the like, in a mediumcontaining a hydrosulflde and an amine selected from the groupconsisting of methylamine, dimethylamine and trimethyamine. Thetreatment may be carried out at a temperature of about 100 C. for about1 hour, these values being varied according to the exact propertiesdesired in the final products. The following examples, in Table Land 11,are given by way of illustration and are not to be consideredlimitative.

The zero strength temperature (Z. S. T.).referred to is that temperatureat which a yarn or similar structurebreaks under a load of 0.5 gram perdenier when in contact with a heated metal surface. The creep value isthe elongation expressed in per cent that occurs in 15 minutes when theyarn or similar structure is heated at a temperature of 100 C. and isplaced under a load of 1 gram per denier. Upon release of this load, thestructure tends to return to its original size and the recovery valuerepresents the percentage of creep elongation recovered in 15 minutes at100 C. after release of the load. Solubility tests were run usingacetone, dimethyl formamide (DMF) and other solvents.

In the following chemical after-treatmen the material being tested wasused in'skein form (50 yards) and was placed in a glass tube with 50milliliters of the particular: agent employed. The reagent consisted ofan amine dissolvedin water which had been saturated with hydrogensulfide or equivalents. The molarities given are based on the percentageof hydrogen sulfide in solution, The glass tube was then sealed and thecontents were heated at the temperatures and for the times indicated. Acontrolled experiment was run as given below. All the yarns or filmsbecame a bright yellow to orange color, the

shade of which varied with the material involved.

Table I Dry Ten- Time Temp. Creep Z. 8. T. Bolubillt in ExampleComposition Treatment 1 (hm) 0.) (Z) 0.) DM

/ 1 (C0ntro]) liolyscrylonitrile 22. 0 147 4. 4 B01. 2 do 9. 8 241 3.9Insol.

8. 0 300 3. 6 Do. 7. 8 311 8. D0. 10. 2 239 3. 4 D0. 8.2 302 a. 5 Do.

o. 2 255 I 3. 0 Do. 10. 6 287 3. 9 Do.

do I 276 Aerylonitrile/methyl vinyl 12. 7 286 Acetone insol. foeotgnetnterpolymer (Control sol.) nu'yloililtrilelsgricae m- 0.86 M 0mm 1 10011.5 251 er v Polyme l cry onitrile... 0.8 M C s Ha 1 100 245 acetoneInsol.

! Unless specified the hydrosulflde is His. 1 Control 136. l Untreated136. 4 Untreated-soluble.

'From the above results it can be seen that treatment according to thisinvention results, in a marked increase in distortion temperatures andin improved thermal stability; For example, the yarns ofpolyacrylonitrile which are untreated have a zero strength temperatureof only 147 C., whereas those treated as described above have zerostrength temperatures averaging 275 C. Furthermore, the creep values arematerially reduced going from 22% for the control to as low as about 6%Iorsome of the modified materials of this. invention. Further, it isnoted that the strength of the materials as measured, for example, bytenacity, has not been disadvantageously affected and that the chemicalproperties, such as solvent resistance, have been materially improved.The structures have,

by the process 01 this invention, been converted from acetoneanddimethylformamide-soluble products to products which are insoluble insuch solvents. I I I It is possible to obtain even better thermalstability or lower creep values by carrying out the treatments with theaqueous solutions of an amine anda thiol compound on structures whichare under tension. This is shown in the following experiments which" aresummarized in Table 11 below. i

A 50-yard portion of a fiber prepared from polyacrylonitrile'was placedin a glass tube conno shrinkage could occur during treatment. This wasalso-heatedin aqueous solution of dimethylamine (1.1 M), the waterhaving been saturated with hydrogen sulfide. The treatment was carriedout for 1 hour at 100 Ctsubstantially in the manner described above.After this treatment the yarn was removed from the reagent, washed andheat treated for 1 hour at 150 C., the yarn still being under tension onthe mandrel. This yarn is designated as yarn B.

The properties 01 yarns A and B are as 1'01- lows:

Table II Dry Te- 7 Cree Recovery, Z. S. T.

Example For 051% Per Cent 0. 2)?

Yarn A 7. 2 as. o 315 a. a Yarn B a. 2 I as. o 320 I 4. 5

From the above figures it can be readily seen that the zero strengthtemperatures are materially higher, being in the neighborhood of 320 C.as compared to the average 015275" C. for yarns which'were treatedwithout application of tension. Furthermore, the creep values have beenmaterially reduced. The yarn treated while under tension had a creepvalue 01' only 3.2% as compared to a value of 7.2% for the control andrecovered to much greaterextent (88%) than did the control (68%). Stillfurther the yarn treated under tension had an increased dry tenacity(4.5 grams per denier) being materially higher than that of theuntreated yarn (3.3 grams per denier).

From the above description and examples. it can be seen that the processof this'invention is applicable, in general, to high polymers containingnitrile groups. The examples have shown the application of thisinvention to materials prepared from polymers obtained by polymerizingvinyl compounds which bear a nitrile group on one or the ethylenecarbons. The po mers contain a plurality of the group:

I I N in which the R. group may be a halogen atom, a hydrogen atom,oranalkyl group. As examples of polymers which may be used, may bementioned 'polyacrylonitrile and polymethacrylonitrile. Furthermore, theprocess is applicable to structures made from the copolymers andinterpolymers of acrylonitrile or, of methacrylonitrile. Such copolymersor interpolymers may be prepared by polymerizing acrylonitrile ormetha'crylonitrile with other monomers such as styrene and methyl vinylketone. In general, polymers prepared by polymerizing acrylonitrile ormethacrylonitrile with any copolymerizable substance having one or moreethylenic linkages may beemployed in the process 01' this invenaueaccaparticular structure with the treating agents of this invention dependupon a number of factors such as the number of nitrile groups containedin the particular polymer from which the structures are prepared. Whilethe number of nitrile groups will vary between wide limits, it isgenerally preferred for electrical laminations and films and fabrics, touse acrylonitrile polymers in which at least 85% by weight of thepolymer molecule is composed of acrylonitrile units. The

amount of acrylonitrile in the polymer may be higher as, for example,100% and it may be very low as, for example, about 5%. Further the speedof the reaction depends on the sensitivity of the particular polymer tothe carrier employed, the

speed increasing with greater sensitivity. For example, if water is thecarrier the reaction proceeds faster with polymers which are watersensitive than it does with polymers which are not so sensitive. This isto be expected since the water-soluble treating reagents of thisinvention employed in aqueous media would penetrate further and comeinto more intimate contact with a water sensitive structure than with anonsensitive polymer.

The preferred treating reagents of this invention are solutions ofmethyl-, dimethylor trimethyl amine saturated with hydrogen sulfide, athiol or a dithiol. Any aqueous solution containing a thiol-containingsubstance and one of these amines or a mixture thereof may be used inthe process of this invention. This invention contemplates the use ofany thiol or hydrosulfide, that is, any compound containing the -SHradical. Thus, under certain conditions other hydrosulfides such assodium, potassium, calcium or barium hydrosulfide may be used to supplythe SH radical. However, the reaction is surprisingly specific inrespect to the amines. Not all amines are operable. For example,triethyl amine and aniline do not give the desired results.

Suitable materials which may be used in place of hydrogen sulfide aremethanethiol, ethanethiol, 1,2-ethanedithiol, propanethiol,hexamethylenedithiol, etc. In addition the particular medium used neednot be an aqueous medium and non-reactant solvents other than water maybe employed. Ethanol, methanol, propanoi, dioxane, glycol, glycerol andsimilar liquids may be employed as media either singularly or asmixtures. Any of the reaction media may, in addition, contain smallamounts of elementary sulfur dissolved or suspended therein.Incorporation of such amounts of sulfur results in acceleration of thereactions. Generally, the media used must be non-reactive toward theparticular polymer, and, in general, any non-reactive solvent may beused as a carrier of the amine and thiol. It is preferred to use acarrier which is a good solvent for the thiol being used and whichpenetrates the polymer structure. to a degree sufiicient to permit morethan superficial surface modification.

As disclosed in copending application of Robert Lowry Brown Serial No.745,633, filed May 2, 1947,

similar results are obtained when aqueous or other solutions aresaturated with hydrogen sulfide and ammonia and used instead of theaminethiol combinations of this invention. Instead of using solutionssaturated with ammonia and hydrogen sulfide it is possible to usesolutions containing ammonium hydrosulfide or solutions in which hasbeen dissolved an ammonium salt, such as ammonium chloride, and'whichhave then been saturated with hydrogen sulfide. Any of the thiols usedin this invention may be used with ammonia or its equivalents instead ofhydrogen sulfide. Modified polymers similar to those described hereinare obtained by using ammonium hydrosulfide.

The amount of the particular reagents used will depend .upon the degreeof modification desired and on the particular polymer being treated. Ifthe amount of agent employed is small, an excessive amount of time isrequired for modification. On the other hand, hydrolysis of nitrilegroups may occur when large amounts are employed. Accordingly,considerable care must be exercised to avoid such hydrolysis. While ithas been described in the above examples that the reagent solutions weresaturated with hydrogen sulfide, it is not essential that such highconcentrations be employed in order for the reaction to take place. Itis preferable, however, that the hydrogen sulfide or the thiolcontaining substance employed be present in an amountexceeding 50% ofthe saturation value in. waterat the temperature of reaction. The use ofconcentrations below this level does not result in formation ofmaterials having optimum properties. Generally, the reaction will occurto give products having improved propertieswhen concentrations of aminesbetween 0.2 M and 2.0 M are employed. The preferred range of amines isbetween 0.4 M and 1.5 M. The use of concentrations of the amines or ofhydrogen sulfide or the thiol-containing substance lower treatingsolution, the reaction temperature may vary from 75 C. to 200 C. (underpressure) with 1e range of C. to C. being preferred.v At highertemperatures, of course, the reaction proceeds more rapidly. Forexample, for a given concentration of reagent, the results obtained by aone-hour treatment at 100 C. can be duplicated by a ten to fifteenminute treatmcntat Of course, this treatment may be carried out oneither yarns, fabrics, films, monofils, molded articles or the like.Also, the material, while being treated, may be in the relaxed state orunder tension so that no shrinkage occurs. In general, the materialstreated under tension will show a reduced elongation and better creepproperties than materials treated in the relaxed state. To improve thecreep properties further, the chemical treating step may be followed byvarious heat treatments .(under tension or relaxed) as described inseveral of the examples. Such heat treatmentsof structures treated bythe process of this invention may be carried out in air, in inertatmospheres, or in vacuum for ter. sulfur present in the polymenitissurprising aeeaeca any period of time or at any temperature necessaryfor a desired result.

The reaction described herein is accompanied by a considerable change inthe color of the treated material. Acrylonitrile polymer, for instance,changes from a light straw-to a deep orange. If the treatment isfollowed by a longheat treatment in air, the color changes to a darkmaroon and finally to black. Other materials described herein changecolor in a similar manner although the color range may differ. Thereason for the color change in the shaped polymers upon chemicaltreatment is not known. The improved properties are due to the combinedeffect of the amine and the thiol. Neither the amine nor thethiol-containing compound, as for example hydrogen sulfide, can effectany material improvement in the thermal properties of the polymer. Thefact that the treated structures are insolubilized suggests thatcross-linking occurs in-the process of this invention. There is nodegradation, nor is there any appreciable elimination from the polymerof small. molecules such as ammonia, water or the like when thetreatment is carried out so that hydrolysis does not occur. While thenature of the reactions is not known, it is believed that the treatmentof this invention produces polymers which are substantially differentfrom those contained originally in the yarns or fabrics.

Sulfur analyses indicate the degree of crosslinking and supply furtherevidence of the fact that new compositions of. matter are produced.Depending upon the length of the treatment and the concentration of thereagents used, among other factors, the sulfur content varies from aslow as 0.2% to a maximum of about These values represent the amount ofchemically combined sulfur. The nitrogen content. however. remainssubstantially constant as is shown in Table III below which also showsthe carbon and hydrogen contents of a typical polymer modified by theprocess of this invention.

Table 111 Analysis in Per Cent Treatment Time C H N S Accordingly, theimprovement in the properties of the structure arises from cross-linkingreactions which produce new compositions of mat- Considering the smallamount of combined that such improvements in thermal stability areobtained.

Surprisingly, the initiatform of the fabrics, yarns or structurestreated by the process of this invention is retained, except for someshrinkage and, in the case of filaments, a'corresponding increase inyarn denier. For example, a yarn composed of eighteen filaments can'beseparated into its component parts after the most drastic treatments ofthis invention. This is true even though the structure may be preparedfrom a low softening polymer (147 C. or lower) and the heating isconducted at temperatures of 150 C. or higher. No coalescing or stickingtogether of filaments occurs, even though temperatures above thesoftening point of the particular polymer are employed. The structuresare not ten-.

dered, nor do they become brittle, but they possess the flexibility andthe pleasant hand of the corresponding untreated materials.

The length of time necessary for the-chemical treatment varies inverselywith the temperature and concentration. For high concentrations. about1.0 M and high temperatures, that is, 150 0., the treatment time may beas short as ten to fifteen minutes. On the other hand, when lowconcentrations (0.25 M) and low temperatures C. to C.) are used,comparable results are obtained only after treatments from about two tofive hours or more. shorter treatment times are preferred for reasons ofeconomy. Using short periods of time with average concentrations of thereagents of this invention and high temperatures, useful products may beobtained without degradation of the polymeric materials comprising thestructures and without hydrolysis of labile groups such as nitrile orester groups.

The process of this invention can be carried out by any of the methodsknown to the art. Batchwise operations may be conducted in which thefabric, yarn or any preformed structure is conveniently immersed in ahot bath containing the reagents. be in skein form or they can be placedon cones or similar storage means which are then immersed in thetreating bath. The process can treating fabric or yarn is to cause it topass slow I ly in a relaxed state on an endless belt arrangement througha heated bath containing the reagents of this invention or tension maybe applied to the structure during passage through the treating bath.

The treated fabrics, yarns or the like of this invention areparticularly useful for electrical laminations and insulation or othersimilar uses in which a high strength fabric, yarn or filament, whichwill retain its shape and strength under high loads or temperatures, isneeded. Such uses are high temperature filtrations, high speed power andconveyor belts, or similar applications.

This invention provides a satisfactory material for these uses since thestrengths of the particular structures at elevated temperatures aregreatly improved by the treatments of this invention. For example, ayarn composed of polyacrylonitrile, after being treated according to theprocess of this invention, has a tenacity of 1.8 grams per denier at 225C. whereas an untreated polyacrylonitrile yarn has a tenacity of only0.7 gram per denier at the considerably lower temperature of 150 C.Further, the percentage creep at C. may be lowered from about 20% toabout only 2% to 4%. This improvement in creep properties is accompaniedby a substantial improvement in the creep recovery properties as isshown in the tables above.

As an additional advantage, structures treated by the process of thisinvention may be further y periods of time to temperatures of about C.

improved in thermal properties by heat treating in air, that is byexposing them for extended Such treatments result in increasing the zerostrength temperature up to about 350 C. and also in a decrease in thepercentage of creep and an increase in the creep recovery.

The process of this invention results in new compositions of matterwhich are polymeric materials and which have improved dimensionalstability at high temperatures and which retain the excellent electricalproperties of the .parent poly.-

Normaliy,

When yarns are used, they canmeric materials. The power factor anddielectric "a power factor of about 0.011 at one million cycles while asimilar fabric of the same material treated as described in Example 1has a power factor of 0.007. Both of the above values are consideredexcellent with regard to electrical insulating properties.

Any departure from the above description which conforms to the presentinvention is intended to be included within the scope of the claims.

I claim:

1. A process for insolubilizing polymeric structures comprisingcross-linking structures prepared from a polymer having a plurality ofnitrile groups attached to the polymer chain by heating, at atemperature of about 100 C. to about 150 0., said structures in acarrier which is inert to said structures and which contains at least50% of the saturation value in said carrier at said temperature of ahydrosulfide and from about 0.2 to about 2.0 molar concentration of anamine selected from the group consisting of methylamine, dimethylamineand trimethylamine thereby insolubilizing said structures.

2. A process in accordance with claim 1 in which said polymer ispolyacrylonitrile.

3. A process in accordance with claim 1 in which said polymer ispolymethacrylonitrile.

4. A process in accordance with claim 1 in which the said polymer is acopolymer of acrylonitrile and styrene.

5. A process in accordance with claim 1 in which the said heating iscarried out from about 35 minutes to about 2 hours.

6. A process for insolubilizing polymeric structures comprisingcross-linking structures prepared from a polymer having a plurality ofnitrile groups attached to the polymer chain by heating,

' at a temperature of about C. to about C. for about 35 minutes to about2 hours, said structures in an aqueous carrier which is inert to saidstructures and which contains at least 50% of the saturation value insaid carrier at said temperature of a hydrosulfide and from about 0.2 toabout 2.0 molar concentration of an amine selected from the groupconsisting of methylamine, dimethylamine and trimethylamine therebyrendering said structures insoluble in solvents for the unmodifiedstructures.

7. A process in accordance with claim 6 in which the said hydrosulfldeis hydrogen sulfide and said amine is methylamine.

8. A process in accordance with claim 6 in which the said hydrosulfideis ethanedithiol and said amine is methylamine.

9. A process in accordance with claim 6 in which the said hydrosulfideis hydrogen sulfide and said amine is dimethylamine.

10. A process in accordance with claim 6 in which the insolubilizedstructures are characterized by improved thermal stability whileretaining the dielectric properties of the unmodified structures.

GEORGE MOORE ROTHROCK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,984,417 Mark Dec. 18, 19342,137,584 Ott Nov. 22, 1938 2,418,942 Morgan Apr. 15, 1947

1. A PROCESS FOR INSOLUBILIZING POLYMERIC STRUCTURES COMPRISINGCROSS-LINKING STRUCTURES PREPARED FROM A POLYMER HAVING A PLURALITY OFNITRILE GROUPS ATTACHED TO THE POLYMER CHAIN BY HEATING, AT ATEMPERATURE OF ABOUT 100* C. TO ABOUT 150* C., SAID STRUCTURES IN ACARRIER WHICH IS INERT TO SAID STRUCTURES AND WHICH CONTAINS AT LEAST50% OF THE SATURATION VALUE IN SAID CARRIER AT SAID TEMPERATURE OF AHYDROSULFIDE AND FROM ABOUT 0.2 TO ABOUT 2.0 MOLAR CONCENTRATION OF ANAMINE SELECTED FROM THE GROUP CONSISTING OF METHYLAMINE, DIMETHYLAMINEAND TRIMETHYLAMINE THEREBY INSOLUBILIZING SAID STRUCTURES.